A conventional power control system connects a power generation system, a battery, and a power grid, and controls a power storage system which supplies power to a load. The control of the power storage system includes a step of determining whether or not the power grid is connected to the load, and a step of determining whether or not power is generated by the power generation system, and operation is performed in one of a plurality of operation modes of the power storage system, based on a determination result of at least one of the generated power amount of the power generation system, a charge status of the battery, a charged power amount of the battery, a power consumption amount of the load, and time (for example, see Patent Document 1).